In recent years, accidents in which hospitalized patients, institutionalized patients, patients in need of nursing care, or the like roll or fall out of bed, and accidents caused by the wandering of patients with dementia have tended to increase year after year. As a method for preventing these kinds of accidents, for example, monitoring systems that use tools such as load sensors, body motion wires, and infrared sensors to detect the actions of a person being watched, such as sitting up, edge-sitting, and leaving bed, have been introduced in hospitals. For example, Patent Literature 1 discloses that a watching region for determining that a patient lying in a bed has performed a getting-up behavior is set in a region directly above the bed including the patient sleeping in the bed. Then, it obtains a variance value that indicates the size of an image region thought to be the patient occupying a watching region in a captured image that includes the watching region as viewed from beside the bed, and if the variance value is less than an initial value indicating the size of the image region thought to be the patient occupying the watching region in the captured image that is obtained from the camera during a state in which the patient is lying in the bed, it is determined that the patient has performed a getting-up behavior.